Sleep: Neural Circuits, Orexin/Hypocretin, Hypothalamus, Neuromodulators, Stress & Cortisol, Sleep Drugs & Ultrasound Technology | Luis de Lecea | #168

About the guest: Luis de Lecea, PhD is a neurobiologist whose lab at Stanford University studies the neural basis of sleep & wakefulness in animals.

Episode summary: Nick and Dr. de Lecea discuss: the neural basis of sleep; sleep architecture & sleep phases (NREM vs. REM sleep); orexin/hypocretin neurons & the lateral hypothalamus; cortisol & stress; circadian rhythms; neuromodulators (norepinephrine, dopamine, etc); sleep across animal species; sleep drugs; ultrasound technology; and more.Related episodes:

* Sleep, Dreaming, Deep Neural Networks, Machine Learning & Artificial Intelligence, Overfitted Brain Hypothesis, Evolution of Fiction & Art | Erik Hoel | #43

* Consciousness, Anesthesia, Coma, Vegetative States, Sleep Pills (Ambien), Ketamine, AI & ChatGPT | Alex Proekt | #101

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* Episode transcript below.

Full AI-generated transcript below. Beware of typos & mistranslations!

Nick Jikomes 3:44

Can you start off by just telling everyone a little bit about who you are and what your lab does?

Luis de Lecea 3:50

Yeah, I'm a professor in the Department of Psychiatry here at Stanford, and my lab has been studying the neural underpinnings of sleep and wakefulness for over 30 years.

Nick Jikomes 4:07

And sleep and wakefulness are really interesting, because in many ways, these are mysterious phenomena. But at the same time, we're all very familiar with these things at a personal level. You know, we all go to sleep and we're all awake. We all know what it feels like to be in a state of high vigilance, in a state of deep sleep and so forth. At a very high level, when neuroscientists talk about behavioral states and brain states, what do those terms mean, and how do you start to think about them?

Luis de Lecea 4:40

Yeah, yeah, that's a great question. So yeah, we talked on the basis that all animals sleep, so all animals have cycles of activity and rest, and we define sleep. Sleep is essentially behavioral state that has one particular a. Property, which is sleep rebound, or sleep homeostasis. That means, as you just mentioned, that when we are awake for, you know, longer period of time, then we feel this urge to go back to sleep, and this sort of sleep drive gets stronger, you know, the longer we've been awake. So that's different from just the circadian cycle where, you know, we just feel that we just want to go to sleep because it's, you know, nighttime. So the neural processes that govern this sleep drive are essentially, are essentially what we're studying when we're studying sleep. And the manifestation of this drive is in changes in the activity of in mammals cortical neurons, and we monitor those changes using an electroencephalogram, which can very broadly, measures cortical, cortical activity. So you know, sleep and wakefulness can be defined objectively with an electroencephalograph, by looking at the shape of the waves, the waveforms essentially in the electroencephalogram.

Nick Jikomes 6:23

So so there's behavioral changes that correlate with these different patterns of brain activity. So obviously, when animals are sleeping, they're not moving around as much. So there's macroscopic things you can see that that you can see in terms of an animal's external behavior, and those are mirrored by general patterns of global brain activity that you can measure with EEG and things like that. That's correct.

Luis de Lecea 6:46

And of course, you know the EEG can be measured only in a few species, in in lower vertebrates, and actually in insects and and other animals. It gets, obviously very difficult or impossible to get an EEG recording. So then we measure sleep strictly behaviorally and and we we look for quiescent periods of quiescence that, again have this property of sleep homeostasis. That means that when we interfere with the with with sleep, we extend the Wake periods, then we see this increased drive to fall asleep, and so

Nick Jikomes 7:36

with that kind of homeostatic regulation. So if you, if you deprive an animal of sleep, you prevent it from falling asleep for longer and longer periods of time, a sleep drive will build up, and then it will sleep more to make up for that. That would imply, right? That there's some, some essential function being served here by sleep.

Luis de Lecea 7:56

That's correct, and we're still trying to figure that out. There are several hypotheses out there as to what the main function of sleep is. Some researchers argue that it's Synaptic scaling. That means that you know that there is an increased synaptic, or communication between neurons. That's what the synapses are. So there's increased communication in the brain that needs to be reset every day so that we don't go just wild in terms of neural activity. So that's the synaptic downscaling hypothesis by Julia Tononi and Cara Chiari, essentially, and and then there's another one. They're actually several hypothesis. Another hypothesis is that sleep is necessary to clean up your brain so that during sleep, there's a there's this increased flow of in the in the brain that allow us to to clear metabolites and toxic substances that accumulate during wakefulness. But there are many others. There are other hypotheses that point to, for instance, you know, DNA repair as one of the main drivers for for sleep and also metabolic recovery. There, there are a bunch of hypotheses, none of which have been proven yet, or only have we only have partial pieces of evidence that that demonstrate that, yeah, there is a maybe, you know that sleep is essential for for some of these, for some of these functions,

Nick Jikomes 9:53

and so, you know, just being agnostic about the evolutionary purpose of sleep, when you look at brain. Activity with EEG in the mammalian brain, where you can measure, measure that activity. What are, what are the major phases of sleep, and what do they look like in terms of general levels of brain activity?

Luis de Lecea 10:17

I'm not sure if I understand that question. So,

Nick Jikomes 10:19

years, yes. So, if you compare sleep to wakefulness, and you compare, say, non REM to REM sleep, you know, these are different behavioral states, and they have very different, you know, there's very different patterns of brain activity you can observe through EEG. What do those actually look like for the different phases of sleep in comparison to wakefulness? Oh, I

Luis de Lecea 10:38

see. So, yeah. I mean, again, that's based on EEG and how we know or what we know about the EEG. So during wakefulness, there is this very complex pattern of activity throughout the cortex, which is what we are measuring in the EEG. And the EEG can be deconstructed into different waves, waveforms we call the alpha, beta and so forth and delta, so yeah, during wakefulness, there's a predominance of alpha and gamma and high frequency activity, okay? And that is in contrast to non REM sleep, which is characterized by a very high contribution of what we call the delta waves, which are slow, slow wave activity, and that is essentially reflects synchronized activity throughout the cortex. That means that many neurons are firing in synchrony during a non REM sleep. And in contrast to that, REM sleep is characterized by very high predominance of what we call the theta activity, which is a activity at the seven to 11 Hertz range, and that is also present, excuse me, during wakefulness. But that is, but again, REM sleep is very, very, is very prominent on that in that that frequency, and of course, REM sleep is also characterized by almost complete muscle atonia. That means that all muscles except for the eyes and and the breathing muscles involved in breathing are essentially lack activity. So, yeah, those are the, you know, the broad picture, the differences between the three and the three vigilant states.

Nick Jikomes 12:44

So there's these very different patterns of brain activity to characterize each of these states. What is like the neural basis for those brain state changes is it just that different neuromodulators are either, you know, being used a lot versus very little, what's actually accounting do we know what accounts for the very different brain states that you can, you can measure with EEG, yeah,

Luis de Lecea 13:08

well, that's we're still trying to figure this out. I mean, it's a very, very complex question, if you want to, if you want me to say that. So we don't know. We don't know, you know, what drives those complex patterns. We cannot really predict them very, very well, in general. And again, that's a very broad picture there. There are a bunch of excitatory transmitters and modulators that are more active during wakefulness than during sleep, but we see all sorts of patterns, both during wakefulness and during sleep. And in fact, there's this phenomenon that was characterized about 10 years ago by vladisovsky, which is called Local sleep. So there is a, you know, this sleep state within wakefulness and and also vice versa, like a wake state within sleep, that that that you can detect in different modules, quote, unquote, in the cortex. So there is not a simple answer to what is turned on during sleep, what is turned off during wakefulness. It's just, it's just very complex.

Nick Jikomes 14:43

So there's both external and internal variables that influence when animals sleep. So So for example, an obvious environmental variable would just be where you are in in the day. So for a diurnal. Animal like humans, we're generally awake when the sun is up, and then we're generally asleep when the sun is down. And you know, we all know that this has some influence on our sleep patterns. We tend to get more sleepy when the sun is down, and we tend to be more awake when the sun is up. But of course, there's internal factors as well. And if you sleep to private animal for long enough, you stay awake for long enough, eventually you'll get so tired that you'll even fall asleep in the middle of the day. And so what are some of these internal factors that build up that drive sleep? Are there, you know, are there? Is there something going on with metabolism, where certain molecules are building up and serving as sleep drivers? Is it sort of have to do with say, you know, the brain using up neurotransmitters or running out of fuel, in some sense, what are some of the major internal drivers of sleep? Yeah, no,

Luis de Lecea 15:47

that's a fantastic, fantastic question. So, yes, so we're again, we're still trying to figure out what those variables are, but, but we know that there are a bunch of them, and that some of them are in running an in conflict at many times, during the day and during the night. So So for instance, you actually mentioned already quite, quite a few of them. So the circadian rhythm, of course, is a main driver. Metabolism is a main driver. Of course, the brain uses 20% of the energy of the whole body. So it's important that the energy allocation to the brain is gets conserved. Then, of course, sleep helps with that. So what else? There's actually a bunch of variables, emotions, and I think we're going to, we can expand on today this. We all know that, you know, very strong positive emotions of excitement and in the wild that would be, you know, access to, for instance, high density of food or resources or the availability of mating partner, so that that stimulus obviously inhibits sleep, because the it's a Very good opportunity to engage in those you know, survival or you know or mating opportunities. So then you know that inhibits sleep, negative emotions. And in the wild, the most negative emotion is to have the risk of survival. So if you have a predator behind you, obviously you're not going to it's not a good idea to fall asleep. So a predator behind you engages the fight or flight response, the stress systems and that very, very powerful, powerfully inhibit sleep. So those variables are integrated in the in several circuits, in the in the central in the in the hypothalamus, and then those conflicts are computed, are sorted out, and then, then the decision to wake up or sleep is being made by that bunch of neurons. And then you know that that information is conveyed to several transmitter systems, including the monoamine the snrp, nefin, serotonin, dopamine and others, and that essentially would wake you up or inhibit, you know, activity to allow for a sleep bed

Nick Jikomes 18:47

and so with sleep deprivation, you know, so we've all had the experience where, you know, we stay up longer than we we should or that we would like to. And you know, gradually you have this build up of a sleep drive, we get more and more tired the longer we've gone without sleep. Is there anything that accumulates over or, you know, with that slow time course in the body that is at least partially responsible for that gradual buildup of sleep? Drive,

Luis de Lecea 19:16

yeah, again, a very interesting question. So if you ask that question 20 years ago, the vast majority of neuroscientists would have answered adenosine. Adenosine is the main molecule that accumulates during wakefulness and is released during sleep. And we all know, of course, that caffeine is an adenosine inhibitor, and that's why one of the mechanisms by which we can stay away for longer. But, you know, research into the past decade or so have has actually tempered that hypothesis and and it's less. There's, it's, the evidence is not so strong, it's, but it is. Way that adenosine is a main pleasure player. There's a lot of work in in metabolic metabolism field and and also in the sleep field, trying to figure out which substances, which metabolites could serve as markers of a sleep drive. And the truth is that it's unlikely that one, there's the single one, a single parameter will drive sleep needs. So it's probably a vector combination, multi dimensional construct that drives sleep. And there are, in addition to the neurometabolic parameters, there are also studies that indicate that there is a shift in the sensitivity of very, very small group of neurons that changes The sensitivity to neuromodulators, and that is this, and that is essentially what drives, you know, sleep pressure. But again, evidence supporting this is only strong in in invertebrates, in Drosophila, in the, you know, the in the flufly, and really not so much in in other organisms. So we still have to be cautious about, you know what? You know how to extrapolate those findings.

Nick Jikomes 21:45

Yeah, so, so there's many variables that that interact or compete with each other, in some sense that the term they get integrated together and determine whether an animal falls asleep or wakes up. And you know, this is a very complex phenomenon. You have. The brain has to keep track of whether or not you're in a rich social environment, whether or not you're safe or there might be predators around, what time of day it is, how long it's been since you last slept, all of these many factors. But you said a decision gets made in the hypothalamus, essentially, which is a deep structure in the brain, and I'm interested in that term, like a decision gets made. It's an interesting way of putting it. And one of the things that's kind of interesting is, you know, despite the fact that wakefulness can change on a slow time, of course, so we can sort of gradually get tired the longer and longer spin since we sleep, ultimately, the the decision or the switch in the brain happens quite abruptly. We fall asleep in a matter of seconds. How does that sort of integration happen within the hypothalamus? And why is it that? Why is it that this sort of very sudden transition ultimately happens where we fall asleep within a matter of just seconds?

Luis de Lecea 22:55

Well, it's really our perception that we fall asleep in a matter of seconds. The brain takes longer to fall asleep, actually. But we could also explain that, you know, that the speed of by which we fall asleep in terms of cortical dynamics, so that's there's essentially what we call, you know, they're emergent properties of very complex circuits, and at one point, when you synchronize, you know enough number of modules of the cortex, then you essentially have to fall asleep. That's, I mean, in very broad terms, that's, that's how we would explain this fast dynamic. But yeah, I mean, indeed, the decision is being made in the hypothalamus, because that's what, actually, that's what lots of neurons do, you know, they they integrate, they compute their inputs, and then, you know, based on those inputs, and of course, that's an online decision. Then the neuron fires or it doesn't fire, and that's a decision that is that has consequences downstream, so that activates or inhibits or doesn't or doesn't activate certain circuits. And that's, that's essentially how it works.

Nick Jikomes 24:13

And so is there a particular circuit or sub region of the hypothalamus where this decision is made? Ultimately?

Luis de Lecea 24:20

Yeah, we've learned that's actually work that we've been conducting for the last 20 something years, almost 30 years. So we identified this group of neurons in the lateral hypothalamus that we call the hypercretin neurons. Other people, other researchers call them orexins, and they're all localized in lateral hypothalamus. They're about, you know, 5000 neurons in the in in rodents, about 50,000 neurons in humans, that are responsible for making that decision. How do we know this? Essentially, because when we don't have those. Neurons. We have narcolepsy. And narcolepsy is a sleep disorder that is characterized by disrupted sleep wake patterns, essentially. Narcoleptic patients don't that lack hypercrete neurons. They cannot, you know, their brain cannot make that decision appropriately. So they don't consolidate sleep. They have serious issues and state boundary control. And

Nick Jikomes 25:31

so these hypocretin neurons, they're in a particular sub region of the hypothalamus. They're important for regulating sleep cycles. If you don't have those, you have narcolepsy, so you have very disordered sleep wake regulation. What is hypocretin or orexin? What type of molecule is it? What does it do at the cellular level?

Luis de Lecea 25:48

Yeah, well, the hypercretins are two orexins are two peptides. They're produced by the same precursor, and peptide transmitters are very widespread in the in the brain and also in evolution, there are molecules essentially that evolved to connect circuits at a slow pace, in a slower time scale than fast translators, and they work by binding to receptors in the postsynaptic what we call the postsynaptic neuron, the recipient neuron and and the you know that the binding of of these neuropeptide to the receptor triggers a whole bunch of intracellular events that result in either excitatory or inhibitory action on the potion optic root.

Nick Jikomes 26:46

So, so these neurons in the lateral hypothalamus, these hypocretin neurons, they are, in a sense, maybe this is simplifying things a bit, but they're the decision makers. They're really important for dictating whether or not the animal will fall asleep or wake up. But with certain exceptions, animals, generally, the entire animal falls asleep. And, you know, the entire brain more or less falls asleep, unless there's exceptions there that we might talk about, how does that decision in one region sort of get broadcast to the entire brain?

Luis de Lecea 27:19

Well, that's because, again, the hypothalamus and that particular region of the hypothalamus where the hypertrophy neurons reside, this region connects to very broadly throughout the brain, and in particular to nodes that are really, really, really important to activate the whole cortex very quickly. And those are what I was referring to before, as the monoamines, norepinephrine, serotonin, dopamine, histamine, and in particular, you know, norepinephrine, that we also described a while ago as being really a very, very powerful and awake promoting transmitter is very, very tightly connected with with hypercreen. So a signal from hypercreen would activate norepinephrine very, very efficiently, and that would essentially light up the brain and wake you up again. In very simple terms,

Nick Jikomes 28:19

I see So, there's these neuromodulators in the brain. They the cell bodies where those neuromodulators are are made. They are in certain parts of the brain, but they send projections throughout large chunks of the cerebral cortex, and, you know, much of the brain. So these hypocretins can broadcast the signal widely by tapping into those neuromodulatory cells which are then going to, you know, sort of communicate broadly across the brain all at once.

Luis de Lecea 28:48

That's exactly what happens. So that's a very, very nice way of describing it. Yes.

Nick Jikomes 28:55

And so you mentioned norepinephrine, which is important here. So I think another name for norepinephrine is noradrenaline. So that might, people might naturally think that's a that's a wake proponent neuromodulator. Is that true? And if so, how exactly does an a neuromodulator like that one promote wakefulness? Is there something sort of special it does to the neurons, or something different than it does in comparison to other neuromodulators.

Luis de Lecea 29:22

Yeah. Well, norepinephrine is indeed a very special transmitter for several reasons, but one being that a lot of a lot of the ruins in the cortex do express these receptors for norepinephrine. So it's a signal that very efficiently reaches to very, very large areas of the cortex and very efficiently stimulates their activity. And what I'm saying efficiently, it's because, you know, at any given time there's a very. A tightly regulated balance of excitation and inhibition in the cortex and norepinephrine can, again, in very simple terms, override, you know, that balance so that it, you know, wakefulness predominates over pretty much anything else that is going on in the cortex at that time.

Nick Jikomes 30:26

Interesting. And so, what? Um, what are some so, so, I would imagine, so when you're awake, nor epinephrine levels are generally quite high, and when you're asleep, I would imagine they're quite low. Um, what is sort of the mix of the major neuromodulators when you're awake versus when you're asleep, say, a non REM versus REM sleep to those must have to change in different ways to define each of those phases.

Luis de Lecea 30:50

Yeah, well, I wish we knew, because we could only until now. Now we have the now we're starting to have the tools to answer that very question, and that is a very, very, very important question. Actually, you know, what's the mix of neuromodulators and transmitters at any given time? So norepinephrine, for instance, the activity of norepinephrine neurons varies actually relatively in very relatively little during wakefulness. So if we count, if we monitor the frequency of activity of norepinephrine neurons during wakefulness, it varies between three to five hertz, so not a whole lot of variation. And during sleep, then it falls to, you know, point 5.2, Hertz, to one. Hertz, okay. And during gram sleep is essentially 0.1, or zero, Hertz, okay. So that's, that's the variation. And on top of that, of course, we have a whole bunch of transmitters, and dopamine, for instance, dopamine has a tonic activity, you know, around again, five hertz also. But one of the characteristics, one of the features of the dopamine is that it has this phasic activity, so just bursting activity, essentially, that is responsive to stimuli or prediction of prediction errors, prediction of errors, as we call it, so and so, the active there's, you know, more complexity added onto that, you know, three to five hertz, a tonic activity in dopamine, serotonin is also has very variable, a very broad dynamic range during wakefulness. And again, we don't know exactly how it modulates transitions to sleep and what is necessary, what is permissive for sleep. But just to go back to your question, so we only know again, single channels so to speak, recording so recordings of a single transmitter at the time. And now we have sensors that have been developed recently by a few of our colleagues that will allow us to essentially have a better description with a multi dimensional vector that allows us to really understand which you know, what the contribution of each single transmitter to brain states.

Nick Jikomes 33:36

And so I want to talk a little bit about sleep architecture so we fall asleep. But sleep itself is not just one state. There's different phases of sleep that have very different characteristics. So there's non REM sleep, there's different phases of non REM sleep, including deep sleep. There's REM sleep, and we've talked a little bit about those already, but what actually, where does this architecture arise from, and what's the significance of transitioning into non REM, and then REM, and then non rem and REM sleep throughout a period of sleeping?

Luis de Lecea 34:12

That is a very difficult question, and I don't think we know the answer. So if so, for people interested in memory consolidation as one of the functions of sleep. They could argue, well, we need to cycle through those phases because that's essentially what you know, that's essentially the way that memory circuits work. Okay? I would argue that that's actually very, not a very solid argument, because their animals have very different sleep architectures, with very different brain configurations. And we can talk about, you know, also very special cases of sleep and but, uh, anyway, so. So we really don't know why there are these phases of sleep. And if we compare, for instance, rodents and humans, we essentially go through very similar phases. But of course, humans have consolidated sleep, so we sleep over eight hours, and we have several cycles during these eight hours, rodents don't have consolidated sleep, so they awake. They wake up a lot during their during their day, which, you know, they're not going to animals, so to sleep during the day. And the other cycles are much shorter and their transitions are much faster. So why do we need those cycles? Why do we need those the sequence of those of the of the phases of the sub states between sleep? We don't really know very well.

Nick Jikomes 35:54

And so you mentioned that there's, there's obviously a lot of variety in nature in terms of what sleep architecture looks like. Some animals sleep much more than others. Some of them, their sleep is more fragmented. You know, they wake up more times, and their sleep is broken up more Are there any? Are there any animals that have sort of bizarre or deviant sleep patterns that you think tell us something important about some of these different phases of sleep and what it might be doing, you know, animals that either, you know, spend a lot of time in one phase or another phase and and just have very different sleep architecture.

Luis de Lecea 36:32

Yeah, I think, well, that is also a fascinating topic in sleep, of course. Yeah, the the ratio of REM sleep versus non compared to non REM sleep varies dramatically across species. We have the I think the extreme is the bats, which are only awake for a few hours, essentially at the dusk, which is when their insects are available, their food is available, and and then they fall asleep for the rest of the day, for 20 hours, and most of this time is REM sleep. Again, we don't know why. I mean, that doesn't it really doesn't fit with memory consolidation, that you need REM sleep for memory. I mean, we don't know why these animals have so much REM there is also some correlation between metabolism and REM sleep, and that was brought up but by Jerry Siegel, so ruminants that have slower metabolism have more REM sleep. Marine mammals don't have much of a much of REM sleep at all. And also, one very peculiar aspect of sleep in these marine mammals is that they sleep with half of the brain, so while the other half is active. So what we call the uni hemispheric sleep. So, yeah. And also, very recently, this example of the penguins. So there was a group that recorded penguins in in the wild, and they showed that up while they're incubating eggs, which is a period of their of their life, or when they're very exposed to predators, so they have to be very vigilant. And they sleep at in two to three second intervals. So it's essentially they are, but they have these sleep episodes like 10,000 times a day. So over overall, they sleep 12 hours. But they sleep only in very, very short sleep bouts. Wow. So

Nick Jikomes 38:50

when they're incubating the eggs, they have to stay on the eggs to keep them warm, especially in the cold environment. Obviously, if they can't move, they're really vulnerable to predators and things. So you're saying they essentially just engage in many, many small little bouts of micro sleep throughout the day. That's

Luis de Lecea 39:04

correct. That's right. Well, these the same animals when they are at sea. They have, they have much more relaxed sleep, they can sleep, and they have much, you know, completely different sleep architecture. And also, you know, there are other examples of extreme sleep, like the flag of birds, for instance, that when they migrate, they are essentially sleepless for a very, very long, you know, two week migration period, right? And also the big changes in in their metabolism as they migrate. And, you know, there are some really incredible adaptive strategies for, you know, under for those those conditions.

Nick Jikomes 39:51

So there's a lot of diversity in nature of sleep architecture, you know. And it's really complicated. So it's going to relate to a lot of things, obviously. The, you know, the lifestyle of the animal when it's foods available. You mentioned, you know, bats, their food is only available one time of day, so they're sort of set up to be awake at that time. They don't need to be awake at other times. They can also hang upside down from the cave, so they're pretty safe, and they can afford to do that, in contrast to the penguins, who have to be prepared to make a run for it or whatever, when they're staying awake to incubate their eggs. So obviously, all these ecological variables that constrain and dictate what sleep architecture actually looks like in a given species, but within a single species, so an experimental species like a mouse, say, Have people been able to manipulate sleep architecture in ways that allow us to understand the contribution of individual parts of sleep to whatever it is that sleep is doing. So, for example, can you selectively inhibit REM sleep but not non REM sleep or vice versa, and see what kind of effects that has on memory consolidation or something like this? Yeah, that has been

Luis de Lecea 40:55

done. And beautiful, beautiful experiments in that in that direction, have shown that, yeah, by inhibiting REM sleep, and not only REM sleep, but actually just the theta oscillation that I was referring to, so that is driven by one brain area in particular during REM sleep is essential for memory consolidation. So that is, you know, those are beautiful experiments. The problem is that it's very hard or impossible to disentangle one part of sleep without affecting the other. So if you affect REM sleep, you're going to necessarily affect non REM sleep. So I would say that the evidence again, supporting that there is something going on during REM sleep for memory consolidation is quite solid. But we were also quite excited a few years ago when this story about memory replay during sleep came about. So that essentially started to, you know, to introduce this concept. Essentially people researchers were recording the activity of neurons in the hippocampus as an animal was exploring a maze and and then there, there are these neurons that are called place cells that fire whenever these animal encounters you know particular space in that maze. So you can actually predict the position of the animal in the maze based on the activity of those neurons. It's actually, it's quite amazing. So there's this group, but MIT Mike Wilson, who showed that during sleep, there was a the sequence of activity that had occurred during wakefulness. The animal was exploring the Maze was replayed during sleep. So, so that provided a very attractive hypothesis that, yeah, so, you know, the animal is replaying so that it remembers where it has been, you know, the day before. And, you know, it potentiates the synapses, blah, blah, blah, blah. So, you know, their whole, whole bunch of theories came from, from the from that observation, but then later studies show that, you know, there's replay also during wakefulness. So it's not sleep is not necessary for that replay. So what is sleep for? Then, you know, that's uh. Again, that's a question we're trying to address. And, you know, going back again to your question. So is REM sleep more important than than non REM? It's still not clear. If you manipulate one, one aspect of sleep, you're I mean, because everything is connected, it's very likely that you're gonna affect others, and then you're gonna, you know, the conclusions are gonna be, you know, much harder, too.

Nick Jikomes 44:09

So, so if you put a mice, a mouse through a maze, it will explore the maze. It will take some trajectory through the maze, and you can essentially see the trajectory the animal is taking by just looking and recording from specific neurons in the brain, and the pattern with which they fire tells you the trajectory the animal took through physical space. And you're saying that during sleep, but also when they're awake and they're just in a state of quiet waking, not really doing anything, the same patterns that define that that fired when they were actually going through the maze will replay in certain circuits of the brain while the animal is disengaged from navigating the maze, either while it's awake or while it's asleep. That's, that's exactly right, yeah. And so, so, you know, I would imagine the natural interpretation of this, when you see this replay during sleep is, you know. Naturally makes you think about dreaming like, Okay, I often dream about things that happened to me recently. Maybe I dream about important things or emotionally salient things more and so maybe, you know, the mouse, if we speculate, is dreaming about what happened to it, and that essentially would correspond, in this interpretation, to the brain replaying certain patterns of activity for the purpose of memory consolidation, is that the basic idea, and if so, it sounds like maybe people actually demonstrated that's really what's going on.

Luis de Lecea 45:32

Well, in regards to dreaming, no, they're actually separate. You know, dreaming is, is a it's a different rearrangement of of activity that occurs during, usually during REM sleep, but also during a deep non REM sleep. So that the replay that that this experiment was referring to, essentially, it's a, it's a, it was a snapshot of the activity of neurons during sleep. And it was surprising that indeed, was almost identical to what the animal had experienced the previous day, so and so. You know, I also became very excited about this, I think, honestly. But again, one has to put in the context of, well, if we only record one channel of activity, and only those cells, those, you know, the place cells we're missing, really the big picture of what is going on during sleep, and that is, yeah, again, that is what we're trying to get, you know, with the current with the current technologies, yes, when

Nick Jikomes 46:38

that replay activity is observed during sleep. What phase of sleep is it in? Is it a non REM sleep or REM? It's

Luis de Lecea 46:44

non REM. Yeah, it's non REM,

Nick Jikomes 46:45

I see. So that would be if REM sleep is most closely associated with dreaming, then, I mean, so, so if presumably, presumably, the rodents were not dreaming in non REM sleep, that's correct. That's correct. Interesting. And so one of the things I want to talk about as well is just like overall sleep depth and sleep quality. So taking those in two pieces, when we talk about sleep death or excuse me, sleep depth, I would imagine that the coarse grained way you measure that is just you. You try and poke the animal or wake it up, and the harder it is to wake it up, the deeper its sleep is. And obviously, you know, again, we've all had experiences like this, where we try to wake sometimes you try to wake someone up, and they wake wake up very easily. Sometimes it takes much more effort to wake someone up. What are the deepest phases of sleep, and do we know anything about what's causing it to be deep?

Luis de Lecea 47:46

Well, the sleep step. Sleep depth is defined by the amplitude of the slow wave. The slow wave activity essentially the delta waves were I was referring to at the beginning. So, and that correlates, as you mentioned, with the the arousal threshold. So you know, the the higher the amplitude of those delta waves, the more difficult it is to wake up an animal or an individual. So that is, again, in general terms, but it's not always the case. And also, there is a very interesting, I would say, disconnect between what we perceive as a good night's sleep, or a sleep where we have slept very deeply, and again, our perception of sleep and the actual, you know, EEG recordings, they sometimes, I mean, most often, they do connect well, but they do overlap well, but in some instances they don't. And so it so happens that there's actually a bunch of people who complain about insomnia. They so they go to their primary doctor. The primary doctor refers them to the sleep clinic or speech sleep specialist. So they do sleep study. And in terms of that, many of these people don't have, they have perfectly normal sleep, but they do complain about not sleeping well. So, and it's not, it's just not an anecdote, it's really many, a lot of people you know, have these disconnect. So, what is it that so are? So what happens then is that the case that we're missing something in the EEG, in. In the electrical recordings that do not really reflect what is going on really in the brain. Is it that we are missing some features of the EEG in the normal protocols of analysis? So we still don't know. But, you know, it looks like, yeah, you know, sleep in these patients seems not to be, almost seems not to be, actually, perfectly fine. So if you there are some parameters that are that are different from in these patients, from the from the healthy controls, yeah. So,

Nick Jikomes 50:42

obviously, so, so if you measure sleep wake cycles with EEG, EEG is really good at defining when you're asleep, when you're awake. Non numbers is REM sleep, but it's a pretty coarse grained measurement, and it sounds like you're basically saying there's many examples of people who their EEG looks perfectly normal. They're going through their sleep wake cycles like people who are well rested do, and yet they say that they're not well rested. So the natural, the natural thing to think there, is that the EGS just not able to tell us the full story. There's something going on that we can't

Luis de Lecea 51:14

see. That's, that's exactly right, yeah, that's so, yeah, with the experimental animals, of course, we have the ability to go deeper at the EEG analysis. We can add more electrodes. We can have, we can really have more, a better idea of what is going on in the brain. We can actually, you know, be invasive and then insert electrodes in the brain the animals, and really see if there's, you know, what else the EEG is not telling us and so. But of course, we cannot, we cannot ask the animal whether you know they feel tired or not, and if they had had a good night or not. So, so that's essentially we're trying to come up with additional measures in the EEG that will be able to tell us in humans, whether this person has is has a healthy, healthy sleep or not.

Nick Jikomes 52:15

And something that's very obvious from experience is that stress affects sleep quality and sleep duration. And you know, when people are really stressed out, they generally report having trouble sleeping, or they wake up frequently things like this. Cortisol is the major stress hormone that you usually hear about as a marker for overall stress levels. How does cortisol change throughout a normal sleep wake cycle for a human. And what is the role of stress and cortisol in sleep?

Luis de Lecea 52:48

Yeah. Well, cortisol has a circadian cycle, right? And so it takes during the during the early hours in morning. And so what does it tell us? So that is, those are again similar to what I told you with the dopamine tone. So cortisol has its day, night oscillation, regular oscillation. But when we have a stressor, then you know that oscillation gets disrupted. So you know, the presence of a stressor

increases dramatically the the release of of cortisol. And that affects, you know, the whole body. Puts the the individual in a state of either ready to fight or ready to flee. And that, of course, affects the immune system, affects just arousal levels, affects the, you know, cardiovascular it affects the whole physiology of the organism,

Nick Jikomes 54:06

interesting. So the other thing that we've mentioned too is there's, there's a lot of external cues that regulate circadian rhythms, besides just, besides just the light dark cycle. So the presence of food can constrain when animals want to be awake versus asleep, and obviously the food is the source of all the energy of the animal. And what's one thing that's interesting to think about, for me is, obviously animals can change their diet. Sometimes their diets change seasonally. Experimentally, you can manipulate an animal's diet. We all have experiences, firsthand experiences, where you know, eating a big meal or eating certain types of food compared to certain other types of food can cause you to get sleepy more than eating other types of food. Do we know about the relationship between food and nutrition and sleep beyond just food availability, obviously, if you're if you're hungry, it's going to be harder to fall asleep. You've all. That experience. Obviously, if you gorge on a huge meal, you could have a sugar spike and then a sugar crash, and that can affect sleep. But, you know, other than those things, can eating certain types of food affect metabolism in ways that that affect sleep architecture. Yeah.

Luis de Lecea 55:14

I mean, other than the what, what you just described, there have been in a number of studies trying to dissect, you know, whether certain lipids, or you know protein, you know protein rich foods, or you know, would affect sleep depth or sleep architecture. And the evidence is not overwhelming. You know, it's, it's a again, you know how we extract nutrients out of the food is, you know, it's a slow process, and it's very regulated. And one would be, I mean, I would be very surprised if, if you know standard, acute changes, or even not you know even, even long term changes in in diet would affect sleep architecture significantly, sleep architecture being really also extremely tightly regulated. Having said that, there's evidence that, for instance, changes in the microbiome do affect, do significantly affect the amount of sleep and that, of course, you know microbiome. Can you know changes with with different diets and so forth, and how you know and how our body extracts nutrients from from those sources. So there, there is, there is a relationship between diet and and sleep architecture, but, but it's not easy to dissect out which components of the diet really are critical to affect sleep, because there there are a bunch of layers in between, and they're difficult to dissect out.

Nick Jikomes 56:59

And how about so? So there's a lot of correlations between sleep deprivation and lots of stuff. I mean, when you sleep derived animals, lots of things tend to go wrong, but my understanding is there's, there's quite a bit of metabolic dysregulation that correlates, and potentially that's even caused by sleep deprivation. So for example, obesity rates correlate with sleep quality. The more sleep deprived you are, the more metabolically unhealthy people tend to be. What do we know about sleep deprivation and metabolic dysregulation? Is there much known about the specific ways that sleep deprivation can influence metabolism?

Luis de Lecea 57:32

Yeah, the general at the general level, yes. And you know sleep deprivation induces insulin resistance. And a whole bunch of you know, especially glucose metabolism is affected by by sleep deprivation, quite significantly, actually, in a way that is similar to, and we can talk more about this. You know, sleep deprivation and aging go are two sides of the same coin in many ways. So the body reacts to sleep deprivation in very much the same way as it reacts to aging. So, you know, as we age, as we get older, our ability to respond to, you know, glycemic challenges is, you know, is impaired. And same thing as with sleep deprivation our, you know, the error rate, so to speak, or the accumulation of errors in our metabolic state also accumulates more rapidly as we age and also with the sleep generation. So there's a really a very good parallel between sleep restriction or not sleeping well, or, you know, insomnia, or, you know, these conditions and aging. So there's no, it's not. It doesn't come as a surprise that there are many indicators of aging that are accelerated in people who don't sleep well.

Nick Jikomes 59:19

Yeah. Well, speaking of aging, I wanted to ask you about sleep across the lifespan. So again, from experience, we know that sleep obviously changes across the lifespan. Newborn babies sleep all the time, more or less, most of the day, and as we get older, we sleep less. Many of us will know that. You know, our grandparents tend to sleep less in a given day than we do. Sleep quality also often changes as we age. In general, people report that their sleep quality seems to decrease as they get older. Can you talk a little bit about how sleep quality and sleep architecture change throughout the lifespan? And I want to maybe differentiate between developmental. Changes that are sort of supposed to happen, or pre programmed versus dysregulation that happens due to aging or accumulated damage or something like that?

Luis de Lecea 1:00:08

Yeah, yeah. No, that's, that's a great question. So yeah, the it's certain that the sleep architecture changes throughout lifespan, and we all know, of course, that infants sleep most of the day. They are essentially growing. They're growing their brain, myelating their synapses. You know, there's a whole really, as you mentioned, the developmental aspect of it, that is for which sleep is essential. And as we reach adolescence, then it becomes more and more important to either just brain development or and or memory consolidation as as as it goes. But of course, at a certain age, yes, there's accumulation of as mentioned, metabolic and also genetic errors that result in in what I would say, you know, disruption of sleep architecture, mostly due to sleep fragmentation. So it's not that older people sleep more. And they do tend to sleep more, but also it's the there's sleep becomes disrupted. And therefore, you know, the sense of, you know, a recovery from sleep is, is decreased. And so we, we published the unimportant paper a couple of years ago, indicating that one of the reasons why, you know, sleep is fragmented in aged individuals is because there are circuits in the hypothalamus affecting the hypercreen neurons that we're referring to before. So these that hypercrete neurons become more excitable as we age, and that means that the brain, quote, unquote, is easier to wake up, and therefore any either spontaneously or due to external stimuli, the sleep becomes more disrupted and therefore